The subject invention relates to the manufacture of polyolefin shrink film using a flat sheet process.
The manufacture of biaxially oriented, heat set polypropylene film using a conventional flat sheet process is well known in the art. An extruded polypropylene web is heated and stretched in the machine direction and subsequently stretched in the transverse direction (MD-TD) and heat set. This orientation improves the film's strength, clarity and gauge uniformity. Similarly, polyesters such as polyethylene terephthalate have been extruded in flat sheet form and biaxially oriented to improve the mechanical properties of the resulting dimensionally stable film; the sequence of orientation here has also been of the MD-TD type.
In contrast, the manufacture of biaxially oriented polyolefin shrink film has most commonly been carried out using a tubular extrusion process. An extruded polyolefin tube is expanded by increasing the pressure inside the tube to stretch the walls of the tube, hence, orienting the film in the transverse direction. Simultaneously, the film is stretched in the longitudinal direction to complete the biaxial orientation of the film.
This tubular process has heretofore been preferred over the flat sheet process for the manufacture of polyolefin shrink film because balanced shrinkage via any flat sheet process was unobtainable. However, limitations such as to inability to manufacture less than about 60 gauge film and film-gauge nonuniformities are prevalent in the tubular extrusion process.